BI-spectral optical detector

ABSTRACT

This bi-functional optical detector comprises:  
     a first active photoconduction detection element ( 1 ) capable of detecting a first range and a second range of wavelengths (λ 1  et λ 2 ), and associated with a first diffraction grating that it uses to couple the light from the first range of wavelengths (λ 1 ) in this first active detection element, so that the first active photoconduction detection element can detect light in the first range of wavelengths (λ 1 );  
     a second active photoconduction detection element ( 1′ ) capable of detecting a first range and a second range of wavelengths (λ 1  et λ 2 ), and associated with a second diffraction grating ( 3′ ) that it uses to couple the light from the second range of wavelengths (λ 2 ) in this second active detection element, so that the second active photoconduction detection element ( 1′ ) can detect light in the second range of wavelengths (λ 2 ).  
     Furthermore, an additional detection element ( 2 ) eliminates the background noise from the first two detection elements.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a bi-functional optical detector fordetecting two wavelengths while eliminating background noise.

[0002] The purpose of this invention is to describe the architecture forcombining two specific previously demonstrated functions of quantic welldetectors, namely the possibility of making bi-spectral devices (Frenchpatent No. 2 756 667) and the possibility of making detectorsintegrating the subtraction function of the continuous component in theactive layer, in other words the darkness current due to thermal causesand the optical scene current (French patent No. 2 756 666). Each ofthese two functions requires a double stack of quantic wells and a3-stage connection. With the invention, this number of stacks andconnection stages can be kept while enabling reading in subtractive modeaccording to three spectral curves, namely λ₁, λ₂ and λ₁+λ₂.

SUMMARY OF THE INVENTION

[0003] Therefore, the invention relates to a bi-functional opticaldetector comprising:

[0004] a first active photoconduction detection element capable ofdetecting a first range and a second range of wavelengths, associatedwith a first diffraction grating that it uses to couple the light fromthe first range of wavelengths in this first detection element, so thatthe first active photoconduction detection element can detect light inthe first range of wavelengths;

[0005] a second active photoconduction detection element capable ofdetecting a first range and a second range of wavelengths, associatedwith a second diffraction grating that it uses to couple the light fromthe second range of wavelengths in this second detection element, sothat the second active photoconduction detection element can detectlight in the second range of wavelengths.

[0006] The invention also relates to a process for making this type ofdetector.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The various purposes and characteristics of the invention willbecome clear after reading the description and the attached figures:

[0008]FIG. 1, showing a quantic well structure for detecting two rangesof wavelengths;

[0009]FIG. 2, the spectral curve for the quantic well structure in FIG.1;

[0010]FIG. 3, an example of the structure of a detector according to theinvention;

[0011]FIG. 4, the absorption and the response induced by the gratings inthe structure in FIG. 3;

[0012]FIG. 5, a top view of the structure in FIG. 3;

[0013]FIG. 6, a matrix of detectors according to the invention;

[0014]FIG. 7, a table of operating modes.

DETAILED DESCRIPTION OF THE DRAWINGS

[0015] The proposed architecture is identical to the architecture for asubtraction component like that described in French patent No. 2 756666, but in this case each of the two active layers is composed of analternation of two patterns of quantic wells (FIG. 1), drawn forabsorption and detection at two wavelengths λ₁, and λ₂ with two spectralcurves Δλ₁ and Δλ₂. The resulting spectral curve is the sum of the twospectral curves Δλ₁ and Δλ₂(FIG. 2). The manufacture of this type ofquantic well detection structure was described by S. Bandara et al., APL72, p.2427 (1998).

[0016] The spectral response of a quantic wells detector is theconvolution of the spectral absorption of the detector and the spectralefficiency induced by geometric resonance of the diffraction gratingused for optical coupling of the incident radiation. Thus, a diffractiongrating resonating at about λ₁ (or λ₂) combined with the previouslydescribed quantic structure will induce a response centered around λ₁(or λ₂) (FIG. 4).

[0017] The principle of the invention is to separate the main pixel withsize a×a into two sub-pixels with size a×a/2, each of these sub-pixelshaving a conventional subtractive structure, but comprising twodifferent coupling gratings (FIGS. 3 and 5).

[0018] Polarization of the lower stage Vref common to all pixels in thematrix remains common to the two types of sub-pixels. On the other hand,polarization of the upper stage that is also common to all pixels in thematrix needs to be duplicated in this case, since each sub-network ofsub-pixels is polarized in a common manner. In this case, connectionsfor the upper stage are achieved by two polarization lines V₁ and V₂ perpixel instead of a single line for a standard subtractive pixel (FIG.5).

[0019] In this case, the device may be used in three subtraction modescorresponding to three spectral curves Δλ₁, Δλ₂ and Δλ₁+Δλ₂ that aredifferent depending on the voltages applied to each of the upperelectrodes (FIG. 6).

[0020] According to one preferred embodiment of the invention, anoptical detector is made on a transparent substrate through whichincident light reaches the detector. This type of optical detectorcomprises a structure like that shown in FIG. 3 on the substrate. Thisstructure comprises a first purely resistive contact layer 5 on onesurface of the substrate, this purely resistive contact layer beingtransparent. A stack of layers 2 is placed on this purely resistivecontact layer, forming a stack of quantic wells capable of detecting tworanges of wavelengths λ₁ and λ₂. A second transparent purely resistivecontact layer 4 is formed on top of this stack of layers. There are twostacks of identical or quasi-identical layers 1, 1′ on the second purelyresistive contact layer, each forming an alternating stack of quanticwells capable of detecting wavelength ranges λ₁ and λ₂, and finallythere are diffraction gratings 3 and 3′ with different structures onthese stacks of layers, such that these gratings each enable couplingeither light at wavelength λ₁ or light at wavelength λ₂, in the stack oflayers (1, 1′ on which it is made). As shown in FIG. 3, the assembly isencapsulated in an insulation layer 6 and connection means pass throughthis insulation layer to reach the diffraction gratings and to formconnection means C1 and C′1 in order to apply voltages +V1 and +V2respectively to diffraction gratings 3 and 3′. Also, at least oneconnection means passes through the insulation layer and reaches thesecond purely resistive contact layer 4 in order to make the commoncontact Cc for reading the photoconduction signal. And finally, aconnection means C2 is formed located on the first purely resistivecontact layer 5 in order to apply the voltage −Vref to the entirestructure.

[0021]FIG. 5 shows a top view of a pair of detection elements like thatshown in FIG. 3. In the example embodiment in FIG. 3, the stack oflayers of quantic wells 2 actually forms two elementary detectors, eachdetector being associated with a detector element 1 or 1′ in order toeliminate the background noise as explained in French patent No. 2 756666.

[0022] The stacks of layers of quantic wells 1, 1′, 2 are active orphoto conducting at the same ranges of wavelengths λ₁ and λ₂. The stacksof layers 1 and 1′ are designed to be more absorbent at wavelengths λ₁and λ₂ than the stack of layers 2. The stack of layers 2 is preferablydesigned so that it is almost non-absorbent. This may be achieved byusing different thicknesses of stacks of layers or by more concentrateddoping of the layers of quantic wells for the most absorbent stacks.

[0023] Since the detector is illuminated by incident radiation on thepurely resistive contact layer 5, the stack of layers 2 receives theradiation first.

[0024] Therefore, the radiation passes through the stack of layers 2,and then through the stacks in layers 1 and 1′, and they are coupledthrough diffraction gratings 3 and 3′, depending on the receivedwavelength (λ₁ and λ₂) in the stacks of layers 1 and 1′.

[0025] If the diffraction grating 3 is designed to couple light atwavelength λ₁, light at wavelength λ₁ will be diffracted towards thestack of layers 1 and will be absorbed or almost absorbed by this stackof layers. Similarly, if the diffraction grating 3′ is designed todiffract light at wavelength λ₂, light will be absorbed or almostabsorbed by the stack of layers 1′.

[0026]FIG. 6 shows a matrix embodiment of a matrix detector according tothe invention. This figure shows four optical detectors like thosedescribed above, as an example. Therefore, each detector comprises twodetector elements each with its diffraction grating. Essentially, thisfigure shows connection means enabling matrix type control of thismatrix of detectors. Therefore, FIG. 6 shows that the connection meansC1 are interconnected through their pin P1 to the control potential V1.Similarly, the connection means C′1 are interconnected through theirconnection pin P′1 at the same control potential V2. The common contactlayer to which a pin P2 is connected, is connected to the referencepotential Vref. Finally, a connection means Cc is placed in the centralpart of each detector in order to connect an individual means ofmeasuring the current for each detector.

[0027]FIG. 6 also shows a control circuit CU to output the referencepotential Vref and control potentials V1 and V2.

[0028] A detection circuit DET receives information v1.2 from thecontrol circuit CU indicating the control mode output by the CU circuit(potentials V1 and/or V2). It also receives read signals output by eachpixel on its common contact means Cc.

[0029]FIG. 7 contains a control table for the detectors thus made. Thistable thus symbolizes a means of controlling detectors. As can be seenin this FIG. 7, a reference potential Vref is applied to the differentdetectors in the operating mode. When a potential V1 is applied to adetector and the potential V2 is not applied, this detector detects thewavelength λ₁. Conversely, if a potential V2 is applied but a potentialV1 is not applied to a detector, this detector detects the wavelengthλ₂. Finally, if potentials V1 and V2 are both applied to a detector atthe same time, the detector detects wavelengths λ₁ and λ₂.

[0030] The normal operating mode may be considered as being the thirdmode in order to acquire the maximum signal. Use in modes 1 and 2alternately is a means of adding a thermometry or passive telemetryfunction. Finally, if there is any optical aggression preventingmeasurements in one of the two spectral windows λ₁ or λ₂, it is foundthat the system can continue to operate and acquire the signal in theother spectral window.

[0031] We will now describe an example embodiment of a detectoraccording to the invention. In order to simplify the description, wewill describe the manufacture of a detector with two detection elementslike the detector in FIG. 3, but this process could be applied for themanufacture of a detectors matrix.

[0032] This process includes the following steps:

[0033] A first purely resistive contact layer 5 is made on a transparentsubstrate (not shown in FIG. 3), transparent to the wavelengths to bedetected.

[0034] A stack of quantic wells is made on this second purely resistivecontact layer, that is photoconducting within wavelength ranges λ₁ andλ₂.

[0035] A second purely resistive contact layer is then made transparentto the wavelengths to be detected.

[0036] A second stack of quantic wells that is photoconducting at thewavelengths to be detected is formed on the second purely resistivecontact layer.

[0037] Two diffraction gratings with different characteristics areformed on this second stack. For example, these two gratings can be usedto diffract light to the second stack of quantic wells, each fordifferent wavelengths λ₁ and λ₂

[0038] Finally, the geometry of each detector is limited by etching.This is done by etching all layers until the first purely resistivecontact layer 5. A chase is also made between two detection elements atthe boundary between the two gratings 3 and 3′ in order to separate thestacks in layers 1 and 1′ from the two detection elements.

[0039] An encapsulation insulation layer 6 is deposited on the assemblythus formed.

[0040] Finally, connection means are made. This is done by making thefollowing holes:

[0041] (a) holes passing though the insulation layer as far asdiffraction gratings 3 and 3′ that are metallized to form connectionmeans C1 and C′1;

[0042] (b) the common connection hole passing through the insulationlayer and the second stack as far as the second purely resistive contactlayer to form the common connection means Cc to create the read signal;

[0043] (c) a hole passing through the insulation layer and reaching thefirst purely resistive contact layer 5 in order to make the connectionmeans C2.

[0044] Note that in the above, diffraction gratings 3 and 3′ may bemetallized for application of potentials V1 and V2.

[0045] Thus the process according to the invention is used to make thedetector in FIG. 3. Several detectors designed in this way should beetched on the first purely resistive contact layer, in order to create adetectors matrix.

1. Bi-functional optical detector comprising: a first activephotoconduction detection element (1) capable of detecting a first rangeand a second range of wavelengths (λ₁ et λ₂), associated with a firstdiffraction grating (3) that it uses to couple the light from the firstrange of wavelengths (λ₁) in this first detection element, so that thefirst active photoconducting detection element can detect light in thefirst range of wavelengths (λ₁); a second active photoconductiondetection element (1′) capable of detecting a first range and a secondrange of wavelengths (λ₁ et λ₂), associated with a second diffractiongrating (3′) that it uses to couple the light from the second range ofwavelengths (λ₂) in this second detection element, so that the secondactive photoconduction detection element (1′) can detect light in thesecond range of wavelengths (λ₂)
 2. Optical detector according to claim1, characterized in that the first two detector elements are made fromthe same layer of photoconducting material.
 3. Optical detectoraccording to claim 2, characterized in that the first two detectorelements are made from stacks of layers forming quantic wells. 4.Optical detector according to claim 2, characterized in that the firsttwo detector elements are made from an alternation of at least two typesof quantic wells, each type enabling detection of a determinedwavelength λ₁ or λ₂.
 5. Optical detector according to claim 3,characterized in that the first detector element is associated with athird detector element (2) separated by a common contact layer or aninsulation layer (4); the fourth detector element is also associatedwith the second detector element, separated from it by a contact layeror an insulation layer; the four detector elements being photoconductingunder the effect of the same wavelength ranges.
 6. Optical wave detectoraccording to claim 5, characterized in that the common or insulatingcontact layers form a single layer (4) and in that the third detectingelement and the fourth detection element also form a single detectionelement.
 7. Optical detector according to claim 5, characterized in thatthe four detecting elements detect the same wavelength ranges. 8.Optical detector according to claim 7, characterized in that theresponse of each first and second detector is significantly greater thanthe response in the third and fourth detectors such that they preferablyabsorb light energy at the wavelength range for which the detector isphotoconducting.
 9. Optical detector according to claim 8, characterizedin that the diffraction gratings are made of a conducting material orare coated by a conducting material and each comprises a first andsecond contact means (C1, C′1) and in that the optical detector alsocomprises: a third contact means (Cc) connected to the faces of thedifferent detector elements that are in contact with the common layer(4); a fourth contact layer (C2) in contact with the third and fourthdetector elements on their faces opposite the common layer; means ofapplying control voltages (+V1, +V2, −Vref) to the said first, secondand fourth contact means (C1, C′1, C2); means of measuring the currentconduction connected to the third contact means (Cc) to measure thephotoconduction of the said detector elements.
 10. Optical detectoraccording to claim 9, characterized in that it comprises control meansof applying a reference voltage (−Vref) to the fourth contact means (C2)and: either a first control voltage (+V1) at the first contact means(C1) to control operation of the first and third detector elements; or asecond control voltage (+V2) at the second contact means (C′1) tocontrol operation of the second and fourth detector elements; or twovoltages (+V1 et +V2) to control operation of all detector elements. 11.Detector according to claim 8, characterized in that it comprises amatrix of detector elements, the fourth contact means (C2) being commonto all detector elements so that the voltage (−Vref) can be applied toall detectors in the matrix, all the first contact means (C1) beingconnected to each other so that the first control voltage (V1) can beapplied on request to all the first detector elements in the matrix, allthe second contact means (C′1) being connected to each other so that thesecond control voltage (+V2) can be applied on request to all the seconddetector elements in the matrix.
 12. Process for making an opticaldetector according to any of the previous claims, characterized in thatit comprises the following steps: formation of a purely resistivecontact layer (5) on the face of a transparent substrate; formation of astack (2) of layers on the said purely resistive contact layer, forminga stack of alternating quantic wells capable of detecting wavelengths(λ₁ , λ₂); formation of a second purely resistive contact layer (4) onthis stack of layers; formation of a stack of layers forming a stack ofquantic wells for the detection of wavelengths (λ₁, λ₂) on the secondpurely resistive contact layer; formation of at least two diffractiongratings (3 et 3′) with different physical structures on the surface ofthe second stack of layers; etching in the resulting assembly of layersuntil the first purely resistive contact layer (5) is reached, of atleast two detector elements, one of the detectors comprising a firstphysical type of diffraction grating and the other detecting elementcomprising another physical type of diffraction grating; encapsulationof the assembly in an insulation layer (6); etching holes in theinsulation layer, passing through the insulation and reaching thediffraction gratings, and metallization of these holes in order to makethe first and second contact means (C1 and C′1); formation of at leastone hole for each pair of detector elements, the said hole reaching thesecond purely resistive contact layer (4) and metallization of this holein order to form the third contact means (Cc); formation of a holepassing through the insulation and reaching the first purely resistivecontact layer (5), and metallization of this hole in order to make thefourth contact means (C2).